Re: [RFC PATCH] mm: thp: implement THP reservations for anonymous memory

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Nov 09, 2018 at 03:13:18PM +0300, Kirill A. Shutemov wrote:
> On Thu, Nov 08, 2018 at 10:48:58PM -0800, Anthony Yznaga wrote:
> > The basic idea as outlined by Mel Gorman in [2] is:
> > 
> > 1) On first fault in a sufficiently sized range, allocate a huge page
> >    sized and aligned block of base pages.  Map the base page
> >    corresponding to the fault address and hold the rest of the pages in
> >    reserve.
> > 2) On subsequent faults in the range, map the pages from the reservation.
> > 3) When enough pages have been mapped, promote the mapped pages and
> >    remaining pages in the reservation to a huge page.
> > 4) When there is memory pressure, release the unused pages from their
> >    reservations.
> 
> I haven't yet read the patch in details, but I'm skeptical about the
> approach in general for few reasons:
> 
> - PTE page table retracting to replace it with huge PMD entry requires
>   down_write(mmap_sem). It makes the approach not practical for many
>   multi-threaded workloads.
> 
>   I don't see a way to avoid exclusive lock here. I will be glad to
>   be proved otherwise.
> 

That problem is somewhat fundamental to the mmap_sem itself and
conceivably it could be alleviated by range-locking (if that gets
completed). The other thing to bear in mind is the timing. If the
promotion is in-place due to reservations, there isn't the allocation
overhead and the hold times *should* be short.

> - The promotion will also require TLB flush which might be prohibitively
>   slow on big machines.
> 

Which may be offset by either a) setting the threshold to 1 in cases
where the promtotion should always be immediate or b) offset by reduced
memory consumption potentially avoiding premature reclaim in others.

> - Short living processes will fail to benefit from THP with the policy,
>   even with plenty of free memory in the system: no time to promote to THP
>   or, with synchronous promotion, cost will overweight the benefit.
> 

Short-lived processes are also not going to be dominated by the TLB
refill cost so I think that's somewhat unfair. Potential means of
mediating this include per-task promotion thresholds via either prctl or
a task-wide policy inherited across exec

> The goal to reduce memory overhead of THP is admirable, but we need to be
> careful not to kill THP benefit itself. The approach will reduce number of
> THP mapped in the system and/or shift their allocation to later stage of
> process lifetime.
> 

While I agree with you, I also had suggested in review that the
threshold initially be set to 1 so it can be experiemented with by
people who are more concerned about memory consumption than reduced TLB
misses. While the general idea is not free of problems, I believe they
are fixable rather than fundamental.

> Prove me wrong with performance data. :)
> 

Agreed that this should be accompanied by performance data but I think I
laid out a reasonable approach here. If the default is a threshold of 1
and that is shown to be performance-neutral then incremental progress
can be made as opposed to an "all or nothing" approach.

-- 
Mel Gorman
SUSE Labs